The present invention relates to an improved signal transmission circuit in a semiconductor integrated circuit such as a dynamic random-access memory (DRAM). More particularly, it relates to a lower-power circuit for driving a load in synchronization with an oscillation signal.
In general, a semiconductor integrated circuit such as a DRAM has an on-chip power-source circuit for simplifying the power-source structure of a circuit in which it is used. In the case of increasing an internal power-source voltage so that it becomes higher than an external power-source potential or in the case of reducing the internal power-source voltage so that it becomes lower than the ground potential, in particular, a charge pump circuit consisting of a capacitor, a rectifying circuit, and the like is used as a power-source circuit.
Below, the structure of a semiconductor integrated circuit in which the charge pump circuit is used as an increased-voltage power-source generating circuit is shown in FIG. 15(a). The charge pump circuit 70 consists of: a pump capacitor 71(Cp); a precharging means 72; and a rectifying means 78. One electrode of the pump capacitor 71(Cp) is connected at the node B to the precharging means 72 and to the rectifying means
The above precharging means 72 has the function of precharging the node B to a specified potential if the potential of the node B is reduced. The above rectifying means 73 has the function of generating an increased-voltage power source by supplying charges from the node B to the output node of the charge pump circuit 70 only when the potential of the node B becomes higher than the potential of the output node, while preventing a reverse current from the output node of the charge pump circuit to the node B when the potential of the node B becomes lower than the potential of the output node.
In the above charge pump circuit 70, if a signal having a specified amplitude is inputted to the node A and the input signal shifts from the low level to the high level, the potential of the node B is increased due to the coupling of the pump capacitor 71(Cp), so that the potential of the output node is increased via the rectifying means 73. Thereafter, if the input signal shifts from the high level to the low level, the potential of the node B tends to become lower due to the coupling of the capacitor 71(Cp) but is clamped at a specified potential by the precharging means 72. The charges supplied from the precharging means to the node B is outputted via the rectifying means 73 when the input signal subsequently shifts from the low level to the high level.
However, in a conventional charge pump circuit, as described above, the following process is repeatedly performed: supplying charges in each cycle so that a specified operation is performed; and releasing the supplied charges in preparation for the next cycle. Consequently, the amount of charge consumed by the charge pump circuit is disadvantageously increased.
By way of example, a specific description will be given below to the operation of the above conventional charge pump circuit of FIG. 15(a), which consumes an increased amount of charge disadvantageously.
Since the charge pump circuit 70 only charges or discharges a pump capacitor Cp from the viewpoint of the node A, the charge pump circuit becomes equivalent to a capacitor of the capacitance C, as shown in FIG. 15(b). A detailed description will be given to the above process with reference to FIG. 10. In the drawings are shown: the charge pump circuit; a time chart for the nodes A and B; the potential difference between the nodes A and B; and the amount of charge accumulated in the pump capacitor Cp. Each of the precharging means and rectifying means of the charge pump circuit is composed of a diode. The power-source voltage for the precharging means is Vcc, which is equal to the power-source voltage for driving the node A. It is assumed that there is no loss in charge caused by the diodes. It is also assumed that, since a charge-pump circuit generally charges an increased-voltage power source having a sufficiently large capacitance, a single operation of the charge pump circuit hardly changes the potential of the output node and the output node of the charge pump circuit is clamped at the potential Vcc+Vp by a constant-voltage power source.
As shown in the time chart of the drawings, a rectangular pulse of the amplitude Vcc is inputted to the node A so as to drive the charge pump circuit, while the potential of the node B varies in synchronization with the potential of the node A. Since the minimum potential of the node B is fixed at Vcc by the precharging means, while the maximum potential of the node B is fixed at Vcc+Vp by the rectifying means and constant-voltage power source, the potential difference between the nodes A and B becomes Vcc and Vp when the node A is at 0 V and Vcc, respectively, as noted in the time chart. The charge appearing at the plate on the side of the node A of the pump capacitor Cp amounts to -Cp.multidot.Vcc and -Cp.multidot.Vp when the node A is at 0 V and Vcc, respectively. On the other hand, the charge appearing at the plate on the side of the node B of the pump capacitor CD amounts to Cp.multidot.Vcc and Cp.multidot.Vp when the node A is at 0 V and Vcc, respectively. As a result, when the potential of the node A shifts from 0 V to Vcc, the charge of Cp.multidot.(Vcc-Vp) is accumulated in the plate on the side of the node A of the pump capacitor, while the charge of Cp.multidot.(Vcc-Vp) is released as an output current from the plate on the side of the node B of the pump capacitor via the rectifying diode. When the potential of the node A shifts from Vcc to 0 V, the charge of Cp.multidot.(Vcc-Vp) is released from the plate on the side of the node A of the pump capacitor, while the charge of Cp.multidot.(Vcc-Vp) from the power source is accumulated in the plate on the side of the node B of the pump capacitor by the precharging diode.
The total amount of charge supplied from the power source to the charge pump circuit is the sum of Cp.multidot.(Vcc-Vp) supplied via the precharging diode and Cp.multidot.(Vcc-Vp) supplied to the node A, while the total amount of charge outputted is Cp.multidot.(Vcc-Vp), so that the charge pump circuit consumes the amount of charge which is double the amount of charge outputted.
Since the precharging means and rectifying means in the charge pump circuit causes a loss in charge in practice, the efficiency of the charge pump circuit is as low as 50% or less. Moreover, in the case of driving the charge pump circuit with a pulse of the amplitude Vcc, the charge of Cp.multidot.(Vcc-Vp) is consumed in each cycle from the viewpoint of the node A, so that the charge pump circuit becomes equal to a capacitor of the capacitance Cp.multidot.(Vcc-Vp)/Vcc.